Fix military timezones in RFC 2822 parsing

[pitdicker]

Currently RFC 2822 formatted datetimes with a single letter as timezone fail to parse, like Tue, 20 Jan 2015 17:35:20 Z.

The existing code had a comment recommended by RFC 2822: consume but treat it as -0000.
But then it would do the opposite and return an error.

I changed it to allow all acceptable military timezones (A-I and K-Z), but keep failing on other input.

[djc]

Where are these military timezones defined, and what do they mean? I think adding support for Z makes sense, but I'm not convinced adding support for other one-letter zones in chrono itself makes sense.

[pitdicker]

To be honest, Z is the only one I care about.

RFC 2822 defines them as:

The 1 character military time zones were defined in a non-standard
way in [RFC822] and are therefore unpredictable in their meaning.
The original definitions of the military zones "A" through "I" are
equivalent to "+0100" through "+0900" respectively; "K", "L", and "M"
are equivalent to "+1000", "+1100", and "+1200" respectively; "N"
through "Y" are equivalent to "-0100" through "-1200" respectively;
and "Z" is equivalent to "+0000". However, because of the error in
[RFC822], they SHOULD all be considered equivalent to "-0000" unless
there is out-of-band information confirming their meaning.

I thought it is best to support the complete format defined by RFC 2822 just to be on the safe side. This is also what time happens to do: https://github.com/time-rs/time/blob/main/time/src/parsing/parsable.rs#L243:L264

[djc]

Fair enough, thanks!